1. Technical Field
The present invention relates generally to optical fibers for use in data storage systems. More particularly, the present invention relates to the suppression of spurious reflections in magneto-optical data storage systems.
2. Description of the Prior Art
In today""s technological society, as the amount of information continues to grow, storage and retrieval of information will play an increasingly important role. In a particular information storage technology known as magneto-optical (MO) data storage and retrieval, a long term goal continues to be improved access to this information. Information access includes the use of a polarized laser light source for reading and/or writing information at a mark of interest on an MO disk. In the case of reading information, MO technology makes use of a magneto-optical effect (xe2x80x9cKerrxe2x80x9d effect) to detect a polarization rotation imposed on a linearly polarized incident laser beam by a surface recording layer at the mark of interest. The polarization rotation (representing the information stored at the mark of interest) is embodied in a reflection of the linearly polarized laser beam and is converted by electronics for readout. Consequently, to accurately read stored information from an MO disk, the polarization orientation of the reflected laser beam should be faithfully conveyed from the MO disk to the readout electronics.
In one proposal, S. Renard and S. Vallette (SPIE Vol. 1499, Optical Data Storage 1991, pp. 238-247) disclose an MO head design that requires three optical fibers to read and write information. Renard""s MO head design is undesirably complex, primarily because of the large number of optical and compensating elements used in its implementation.
In an approach that uses polarization maintaining (PM) optical fiber, the intrinsic properties of the fiber can be made to preserve the optical polarization as required for MO recording. Because PM optical fiber generally exhibits birefringence, (i.e., a different refractive index that different polarization orientations experience), external stresses or temperature variations may function to induce unwanted phase fluctuations between the two polarization modes of the PM optical fiber. Consequently, any information conveyed by the polarization rotation as it propagates through the PM optical fiber may also be affected. A proposal for passively eliminating phase fluctuations caused by the properties of PM optical fiber is discussed by M. N. Opsasnick in SPIE Vol. 1499, Optical Data Storage 1991, pp. 276-278. As in the design of Renard and Vallette, the Opsasnick MO head and actuator arm design is limited by its physical size, mass, and the number of optical elements required. In general, the greater the number and mass of the optical elements used to access information in an MO data storage and retrieval system, the slower the speed at which the information may be accessed, the lower the tracking bandwidth becomes, and the lower the track density that may be read or written.
A drawback of PM fiber relates to the undesired laser noise that arises due to spurious reflections from a front end face and/or back end face of a fiber in a magneto-optical data storage and retrieval system. In particular, the undesired laser noise occurs when the spurious reflection co-propagates with a reflected laser beam and when the spurious reflection and the reflected laser beam assume approximately the same spatial distribution, thereby degrading the signal-to-noise ratio (SNR) of the Kerr signal.
A third approach to data storage based on flying head optical technology with free-space optical propagation to and from the head is proposed by N. Yamada (U.S. Pat. No. 5,255,260). In particular, Yamada discloses an optical head arrangement that requires one stationary laser/detector package per head, with the head placed on a linear actuator for movement across a disk surface. Yamada does not address the problems associated with vertical runnout of the disk or the associated degradation of the optical spot size. Although Yamada provides access to a plurality of phase change optical disks, the number of optical disks that may be operated within a given volume, as well as the performance characteristics associated with the optical disks, is inherently limited by the excessive number, size, and cost of the required optical and mechanical components.
What is needed is an optical system and method that improves upon conventional efforts directed towards data access. What is also needed is an optical system that can reduce head weight and size, improve disk access time, require fewer optical components, increase the number of storage disks that may be operated within a given volume, and be inexpensive and easy to manufacture. In addition, there is a need for an optical system that can transmit light between a laser source and a storage location of an optical drive with a sufficient signal to noise ratio (SNR).
In accordance with the present invention, there is provided a low noise apparatus and method for transmitting optical information between a laser source and a storage location. The low noise apparatus and method advantageously reduce head weight and size, provide a set of low profile optical paths, improve information access times, require fewer optical components, and increase the storage capacity available within a given volume, as compared to conventional approaches.
In accordance with the present invention, optical information is transmitted along an optical path that includes a first optical fiber located between the laser source and a selected storage location. Alternatively, optical information is transmitted along an optical path selected from a set of optical paths that includes the first optical fiber and a set of second optical fibers. In one embodiment, the optical paths are confocal optical paths, the storage location includes a set of magneto-optical storage disks, and the optical fibers are single-mode polarization maintaining (PM) optical fibers. Each of the optical paths may be coupled along a respective positioning arm to a respective flying magneto-optical head. The respective positioning arms are, for example, rotary actuator arms. Each of the optical paths is typically positioned by its respective rotary actuator arm so as to selectively access a reflection of the source of polarized light from a selected magneto-optical disk. The set of optical paths may further include an optical switch that selectively routes information between a fixed optical module (containing the source of polarized laser light along with a photodetection means) and the set of magneto-optical storage disks.
In another aspect of the present invention, various noise reduction techniques are provided by substantially decreasing or eliminating spurious reflections (or the effects thereof) at the end faces of the optical fiber. These noise reduction techniques may be applied if the laser source is, for example, a Fabry-Perot (FP) laser or a stable single-frequency laser source such as a distributed feedback (DFB) laser. In particular, spurious reflections (or the effects thereof) may be eliminated at the fiber front (launch) end face near the laser source and at the fiber back (head) end face near the storage media.
To eliminate the effects of a spurious reflection from the fiber front end face, the laser source may be modulated at a particular frequency that depends on the length of the optical fiber. As a result, the spurious reflection from the fiber front end face is time-separated from the main signal-bearing beam returning from the storage media.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber front end face is eliminated by coupling the fiber front end face to a material having a refractive index equal to the refractive index of the core of the optical fiber. The material may, for example, be formed from epoxy, fluid, or other suitable materials.
In another embodiment in accordance with present invention, the spurious reflection from the fiber front end face is eliminated by coupling the fiber front end face to a cover slip having a refractive index equal to the refractive index of the core of the optical fiber. The cover slip may, for example, be formed from glass or other suitable materials.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber back end face is eliminated by coupling the fiber back end face to a fluid or epoxy having a refractive index equal to the refractive index of the core of the optical fiber.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber back end face is eliminated by coupling the fiber back end face to a coreless or multi-mode fiber portion having a refractive index equal to the refractive index of the core of the optical fiber. The coupling of the fibers is carried out, for example, by fusion splicing.